102

ROAD ENGINEERING

Where, on circles of limited size, the outer kerb is concentric with
the inner, and traffic is less than the capacity of the roundabout, the
tendency will be to cut in or travel on the inside lane, so that the
outer carriageway is little used.

1.
2.
3.
4.
5
6.

Diameter of island (ft.).
Road width around island (ft).
Total width between outer kerbs (ft)
Maximum converging angle (degrees)
Peak capacity (vehicles per hour inc. cycles) up to...
Remarks.

60
SO
120
85
2,500

75
30
135
59
3,000

100
30
160
55
3,500

105
40
185
59
4,000 I
Alternative layouts

150
30
210
40
4,000 I
of which the larger

140
50
240
53
5,000 f
island is to be

180
40
260
40
5,000 J
preferred

240
50
340
40
6,000

Note.—The maximum volume in col. 5 should be used for an estimated
future volume which should be 75% of the maximum capacity.
This condition may be avoided by straightening the outer kerb
even though this causes a greater width of carriage-way than the
minimum at the narrowest point. With large circles the trouble is
less likely to occur, Fig. 63.
An example of a "split " traffic island is shown in Fig. 64,
Level of Centre Island.
A bold kerb, preferably with sloped face, should define the cir-
cumference of the circle; double kerbs are even better. It is good
practice to not raise the soil, turf, or shrub more than about 3 ft. 6 in.
above road level. This prevents dazzle from opposing traffic, but
allows visibility for all drivers across the circle itself. With very
large circles this question is not so important.
Direction Signs.
Ample direction and reflector signs should be placed on the radial
*oads and the island to assist traffic; the usual warning and approach
signs are essential.
White lines are not usually necessary on the carriage-way either
iround the circle or at the entrances or exits.
3uperekvaiion on Circles.
The crossfall to be provided to the road surface of a roundabout
s an Important feature for assisting the movement of traffic.